A bloody but little-known chapter of Mormon history gives director Christopher Cain plenty of ammo for a stinging attack on religious extremism in "September Dawn."

A bloody but little-known chapter of Mormon history gives director Christopher Cain plenty of ammo for a stinging attack on religious extremism in “September Dawn.” With one eye fixed on current world events, this handsome indie Western damningly recounts the 1857 slayings of 120 settlers passing through Utah, but the didactic presentation, grim speechifying and tacked-on love story all signify a less-than-healthy regard for the audience’s intelligence. Limited commercial prospects will depend on the film’s ability to exploit Mormon outrage — the louder, the better — with its angry and contentious view of a still-disputed tragedy.

Taking a page from “Titanic” and “Pearl Harbor,” Cain and co-scenarist Carole Whang Schutter have cooked up a doomed romance as an easy point of entry for young auds in particular. Pic otherwise aspires to a certain degree of authenticity, especially in its climactic re-enactment of the Mountain Meadows Massacre, details of which were drawn from the written confession of John D. Lee (the only person ever convicted and executed for his role in the atrocities).

The film goes one step further, alleging a cover-up of the fact that the killings were ordered by Lee’s adoptive father, Brigham Young. As played by Terence Stamp with an ugly, ever-present scowl, the man who presided over the Church of Jesus Christ of Latter-day Saints from 1847 until his death in 1877 emerges onscreen as a pitiless, self-righteous monster, with a band of fanatical followers ever in his grip.

Chief among these followers is Jacob Samuelson (Jon Voight), the bishop and leader of a Mormon community in southern Utah. When a California-bound wagon train arrives, led by kindly Capt. Fancher (Shaun Johnston), Samuelson allows the settlers to rest in the nearby Mountain Meadows for two weeks, but secretly instructs his sons, Jonathan (Trent Ford) and Micah (Taylor Handley), to keep a close watch on these “Gentile dogs.”

Unlike Micah (a naive but precocious little polygamist), Jonathan is the story’s token Mormon with a conscience — good-looking, sensitive, the only man in town without a wife or two. He easily befriends the pioneers, employing his devastatingly sexy whisper to tame first a wild stallion, then the heart of a fetching blonde lass, Emily (Tamara Hope); she, in turn, helpfully quotes Scripture (“Judge not, lest ye be judged”) that leads Jonathan to question his father’s absolutist ways.

First half of “September Dawn” is middling frontier soap opera, marred by occasionally stilted dialogue and thesps who aren’t always at ease with the period diction. But the pic darkens irrevocably as Samuelson and the townsfolk descend into maniacal bloodlust: Convincing themselves that the settlers were involved in the murder of Joseph Smith (played by Dean Cain, the helmer’s son, in gray-toned flashback) and other Mormon prophets, they demand justice in the form of “blood atonement.”

A violent montage, overlaid with excerpts from Young’s own writings, elucidates this chilling doctrine — that sinners must be redeemed not through faith in Christ, but through their own bloodshed.

Perversely, then, Young, Samuelson and their hysterical ilk see themselves as mercy killers, meting out salvation in the form of death. At Samuelson’s insistence, Lee (Jon Gries) pressures the local Paiute Indians to attack the wagon train, but the resulting siege merely sets the stage for a bloody coup de grace that leaves no doubt about the true villains of the piece.

Auds may rightly recoil from the graphically staged finale, with its almost fetishistic focus on the guttings of women and children, all shot in slow-mo and edited together in a flurry of impressionistic dissolves. It’s not torture porn; it’s massacre porn.

Cain’s portrait of sanctimonious hypocrisy is certainly unflinching, encouraging viewers to draw their own parallels between modern-day and Latter-day terrorists (and carefully emphasizing that the Utah slayings took place on Sept. 11, 1857). But the pic is ultimately less interested in understanding its Mormon characters than in demonizing them, and a ham-fisted obviousness undermines scene after scene (such as one that juxtaposes the travelers’ heartfelt prayers with Samuelson’s fire-and-brimstone denunciations). Nor does the pic convey any insights into the psychology of extremism, aside from some choice moments in Voight’s persuasively complex performance.

Well-scouted Calgary locations are beautifully photographed by Juan-Ruiz Anchia, showing off sun-dappled pastures and mountainous landscapes to good effect, though a dearth of establishing shots deny the pic a coherent sense of geography. William Ross’ elegiac, sometimes overpowering score provides sturdy support throughout.